In machines having a rotatable shaft, it is often desirable or mandatory to eliminate vibration in the shaft caused by shaft imbalances or imbalances in the load being rotated by the shaft. Shaft vibration is a particular problem with precision grinders, saws, lathes and other manufacturing equipment, as well as many other rotating devices, such as turbines and ventilation fans.
In the past, it has sometimes been possible to dynamically balance a shaft to eliminate vibration by fixedly attaching to the end face of a shaft free end protruding from the machine being balanced a device containing a pair of rotatable balancing masses carried in a case. The case also contains a pair of electric motors, with each motor being used to independently rotate one of the balancing masses within the case. The balancing masses are rotated to a position at which the combination compensates for the shaft imbalance and shaft vibration is eliminated. Such a device usually operates in conjunction with a vibration sensor or transducer which senses/measures the vibration of the machine and an electronic controller which analyzes the sensor information and provides electrical command signals to the motors within the case. These signals cause the motors to rotate the balancing masses into the desired position to eliminate imbalance.
As noted above, since the case is fixedly attached to the shaft, and the shaft is rotating at the rotational speed of the machine which, by way of example, can be several hundred to several thousand revolutions per minute, collector rings and slide contacts must be used to make electrical connections between the stationary wires that conduct these command signals from the controller and the rotating case containing the motors. Not only does this present the problem of maintaining good electrical contacts between stationary and moving components, but the problem is made even more difficult because of the environment in which such prior art balancers are frequently used. In particular, when used on grinding wheels, lathes and saws there is frequently machine oil or water being sprayed about as a lubricant and forming a mist which can contaminate or corrode the electrical contacts unless adequate seals are used between the stationary component and the rotating case. Of course, should the seals leak, electrical contact can be degraded or lost. With grinders, lathes and saws, there is also frequently dust or cuttings produced by the manufacturing process being performed. Even in devices such as turbines and ventilation fans, ordinary dust and other particulate contamination can create contact problems.
Another problem with prior art balancers relates to the large weight of the balancing masses required to balance most shafts. Since the balancer case is located at the end face of the shaft free end, it is positioned an appreciable distance from the load being carried by the shaft and the point where imbalance is likely to occur. Because of this distance, larger balancing masses are required than would be the situation if the balancing masses were placed immediately adjacent to the load or even within the shaft itself radially inward of the load. Placement of the balancer within the shaft can be accomplished with relatively large diameter machine shafts by mounting the balancer within a counterbore in the shaft. This is, of course, only possible with shafts having a sufficiently large diameter that such a counterbore can be formed without significantly reducing the strength and integrity of the shaft. Even with machines having a sufficiently large diameter shaft but without the necessary counterbore being formed therein at the time of manufacture, such internally mounted balancers are generally not desirable. Because of the expense and difficulty of machining a counterbore in an existing shaft, the fact that doing so could interfere with the operation of the machine, and the potential problems such could cause, internally mounted balancers are not used to retrofit existing machines. As such, the need continues for an externally mounted balancer.
As noted above, the relatively large weight of the balancing masses, when added to the weight of the case, electric motors and other components of the balancer, and when mounted at the shaft free end sometimes itself creates an imbalance. The weight creates an axial moment in the shaft, which is a particular problem with high precision grinders being used for side cuts. The axial moment can result in lateral wobble in the grinding wheel which reduces the precision of the side cut and produces swirl shaped marks in the surface being ground.
Another problem with prior art balancers which are mounted at the shaft free end, particularly with grinders, is that the balancer must be removed from the shaft each time the grinding wheel is changed, since the grinding wheel must be pulled off from the free end and a new replacement grinding wheel put on from the free end. This significantly increases the time and expense involved in changing a grinding wheel because of the extra time involved to remove and replace the balancer, and because of the balancer calibration and other start-up problems which are frequently encountered.
In addition to the other problems discussed, prior art balancers utilizing electric motors tended to have higher than desired maintenance and repair problems, and the manufacturing cost is higher than desired because of the motors. It has also been discovered that the centrifugal force to which the electric motors are subjected during high speed operation within the case as it rotates affects the output rotational speed of the motors, thus causing inaccuracies in the balancing process. Since electric motors are utilized, cables used to conduct the command signals, get in the way and are themselves subject to damage.
It will therefore be appreciated that there has been a significant need for externally mounted dynamic balancers which overcome the aforementioned deficiencies in prior art balancers. The present invention fulfills this need and further provides other related advantages.